A) if you are getting a refund the IRS will pay YOU interest due to the delay in processing
OR
B) if you owe you can pay the IRS anytime by going online or mailing in a payment. If you are talking about any penalties or interest on the balance due then it is calculated starting with the date the return was mailed and not processed. If you already sent/made the payment and you got a bill then you can either ignore it and when the IRS does finally process the return it will all balance out ... OR mail in proof of payment with a note saying you don't owe anything.
